How Plants Perpetuate Their Kinds 299 



fact they do in fertihzation, then a certain proportion of those con- 

 taining yellow determiners will unite with others containing yellows, 

 and green will be excluded from the resulting individuals ; a certain 

 proportion of greens will unite with greens, thus excluding yellow; 

 a certain proportion of greens will unite with yellows, and a cer- 

 tain proportion of yellows will unite with greens. These propor- 

 tions (allowing for the fact that the yellow-green and the green- 

 yellow are indistinguishable) will be precisely those actually 

 found by the law to exist, as above described, and precisely those 

 shown in our diagram. 



There remain a few miscellaneous matters, connected with 

 reproduction, which must be considered before this chapter can 

 be brought to a close. 



It is almost invariably the case that an egg-cell must be fer- 

 tilized by a male cell before it will grow to a new plant, but a few 

 exceptions are known. In some few plants of the Composite 

 family, and in the Plant Lice among Insects, the egg-cells grow 

 directly into new individuals without any fertilization or other 

 connection with the males, — a phenomenon appropriately called 

 parthenogenesis. It is a kind of asexual growth of the egg-cell, 

 comparable with the growth of a tiny bud; and possibly the es- 

 sential meaning of the process lies in its asexual character. It is 

 conceivable that these particular kinds of plants and animals have 

 reached the highest practicable stage of adaptation to the con- 

 ditions around them, in which case it would be natural for them 

 to preserve their characteristics unchanged by resorting to asexual 

 propagation, using the method which entails least disturbance to 

 existent structures and habits. 



In my account of fertilization I showed that the pollen grain 

 when it enters the embryo sac contains two separate nuclei, only 

 one of which is needed to fertihze the egg-cell. The fate of the 

 other is most pecuhar, for in some plants at least, and probably 

 as a rule, it fuses with a nucleus belonging to the embryo sac 

 itself. The resultant cell grows into the mass of food substance 



